One prior art well-known process for preparing a silicone block polymer is by hydrolysis of a both end reactive diorganopolysiloxane of the following formula: ##STR1## wherein R is an alkyl radical, X is a chlorine atom, hydroxyl radical or alkoxy radical, and t is an integer of at least 1 and a polyfunctional silane of the following formula: EQU R.sub.u SiX.sub.4-u
wherein R and X are as defined above and u is equal to 1 or 2. Undesirably, this process completes with a part of the both end reactive diorganopolysiloxane unreacted, and coatings of the cured polymer are susceptible to stain.
It is also known to subject the above-defined both end reactive diorganopolysiloxane and another silane to cohydrolysis in the presence of an acid catalyst. Due to cracking of the both end reactive diorganopolysiloxane, the resulting block polymers are sometimes less desirable and less improved in low-temperature resistance and flexibility. For example, Japanese Patent Application Kokai No. 125466/1981 proposes a coating composition comprising colloidal silica, RSi(OH).sub.3, and the both end reactive diorganopolysiloxane mentioned above, which is described as forming coatings having improved crack resistance. This composition, however, does not achieve a satisfactory improvement in crack resistance since the both end reactive diorganopolysiloxane is less reactive.
For the purpose of improving the water repellency, mold release, and lubricity of room temperature curable silicone resins, it is also known to add a diorganopolysiloxane of the following formula: ##STR2## wherein R is as defined above and R' is an alkyl radical of the same type as R or different type from R. The resulting polymers are less curable, leaving the problems of stickiness and staining due to bleeding of unreacted oily diorganopolysiloxane.
Further, Japanese Patent Application Kokai No. 145815/1985 discloses a silazane base block polymer, which suffers from many problems. Since the polymer is prepared using ammonia, chlorosilane and organic solvent, there is formed an ammonium chloride by-product which has to be removed by filtration in a sealed environment. The silazane base block polymer needs careful handling because it is sensitive to moisture.